Field of the Invention
The present invention belongs to the field of microwave technique, in particular to the measurement technique for determination of superconductor surface impedance in millimeter and sub-millimeter wavelength ranges.
Description of the Related Art
Accuracy and sensitivity of substance microwave properties measurement depend on quality (Q)-factor of the resonator. Important peculiarity of the resonator is possibility to measure samples in the forms of small ones or thin films without their pretreatment. In case of studying high temperature superconductors (HTS), measurements of HTS film microwave properties depend on amplitude of the microwave field, i.e. measurements of HTS nonlinear properties are very important.
Conductors including superconductors are characterized by the microwave surface impedance ZS=RS+jXS, where RS is the surface resistance and XS is the surface reactance. Measurement of surface impedance ZS is a technical task to determine the microwave properties of superconductors, and a research task in a measurement technique for studying electron system in the above mentioned materials.
Measurement of the surface resistance RS of the superconductor is a difficult task, because its value is very small. For example, even in Ka-band at liquid nitrogen temperature (77 K) RS=5-7 Ohms for high-temperature superconductor YBa2Cu3O7-δ. By increasing the frequency, RS increases as the square of frequency (f2), what indicates possibility to increase sensitivity of measurement at the higher frequencies. Unlike normal conductors with normal skin effect, for all superconductors the surface reactance is unequal to the surface resistance and must be measured also [Physical Properties of High Temperature Superconductors V, Editor Donald M. Ginsberg//World Scientific Publishing Co. 1996, 471 p].
With respect to the superconductors, even if surface reactance XS thereof is larger than surface resistance, and at temperatures significantly below the critical one, Xs>>RS, XS remains a small quantity. Nonlinear impedance properties of superconductors, when the surface impedance is a function of the intensity of the microwave field or its power, have great scientific and practical importance.
For the purpose of measurement and study of the superconductor microwave impedance properties, resonator methods are commonly used because they provide greater sensitivity and accuracy. As a rule, the value of Q-factor and resonant frequency of the resonator are measured at weak coupling of feeder lines with the resonator, and the value of Q-factor and resonant frequency of the resonator are close to the resonator eigen characteristics. Q-factor and resonant frequency contain information about electrophysical characteristics of the sample [Zhi-Yuan Shen, High-Temperature Superconducting Microwave Circuits.—Boston-London: Artech House, 1994, 272 p.]. The difference between the known methods is only in the types of resonators used, forms of samples, ways of their placement in the resonator and methodology of study.
When measuring the nonlinear properties of superconductors, it is important to strengthen coupling of the resonator with transmission feeder lines. Here it is necessary to know the value of a coupling coefficient in order to find the resonator eigen Q-factor.
Usually, the same resonators are used to measure RS and XS. The most important characteristics of all methods of impedance properties measurement are their accuracy and sensitivity. Both properties depend on, which part of the total losses in the resonator are the energy losses in superconductors (i.e., in the sample under test). The energy losses are very small in superconductors compared with losses in normal metals, so normal metals are desirable to exclude completely.
For developing measurement technique of microwave surface resistance, cylindrical dielectric resonators with conducting endplates excited with lower modes have been suggested. Superconducting films on a dielectric substrates were such conducting endplates [Mazierska J. and Wilker Ch. Accuracy issues in Surface Resistance Measurements of High Temperature Superconductors using Dielectric Resonators (corrected), IEEE Trans. Appl. Supercond. —2001.-vol. 11, No 4.—P.4140-4147]. The abovementioned films are the subject of measurement.
The disadvantage of this device is the practical impossibility to use the dielectric resonator with lower modes in the millimeter and sub-millimeter wavelength ranges through excessive reduction in the size of the resonator and the associated difficulty of effective coupling of the cavity with transmission lines, which further reduces the accuracy and sensitivity of the measurement method.
The mentioned drawback is eliminated in the measuring whispering-gallery-mode resonator, which contains the sample under study and presents a dielectric cylindrical disk as resonating body with one or two flat bases perpendicular to the axis of rotation, in which the endplates of material with high electrical conductivity are installed, and which is equipped with a coupling unit containing a transition from rectangular standard waveguide to the feeder line in the form of dielectric waveguide [Cherpak N, Barannik A, Prokopenko Yu, Filipov Yu, Vitusevich S. Accurate Microwave Technique of Surface Resistance Measurement of Large-area HTS Films using Sapphire Quasioptical Resonator//IEEE Trans. on Appl. Supercond.—2003.-vol. 13, No 2.—P. 3570-3573]. The resonator is excited with higher modes, namely, whispering gallery modes. The device allows measuring also the temperature dependence of the surface reactance of superconducting films [Cherpak N. T., Barannik A. A., Prokopenko Yu. V., Vitusevich S. A. Microwave Impedance characterization of large-area HTS films; Novel Approach, Superconductivity Science and Technology, vol. 17, No 7, p. 899-903, 2004].
The disadvantage of this device is the need to use two films in one act of measurement, so the number of acts of measuring individual characteristics of films increases.
The closest analogue on the technical essence is the measuring whispering-gallery-mode resonator, which contains the sample under study, presents a dielectric resonating body with one or two flat bases perpendicular to the axis of rotation, in which the endplates of material with high electrical conductivity are installed, and which is equipped with a coupling unit containing a transition from rectangular standard waveguide to the feeder line of the coupling unit [Device for measurement of superconductor surface impedance//Barannyk O. A., Bunyaev S. O., ProkopenkoYu. V., Filipov Yu. F., Cherpak M. T. Declarative patent for utility model, UA, 16620U, G01R 27/04, 2006]. The measuring resonator is intended for measurement of microwave surface impedance of superconductors, which are made in the form of endplates of high conductive materials. For the purpose of measuring the individual characteristics of the film in a single act of measurement, the measuring resonator is made as a dielectric resonating body, one of the bases of which abuts to the endplate, which is the superconductor film under test.
In the resonator, whispering gallery modes are excited, so one can increase the size of the resonator in the millimeter wavelength range. The coupling unit of the measuring resonator with a transition from standard rectangular metallic waveguide to the feeder line of coupling unit is made in the form of dielectric waveguide, located near the lateral surface of the dielectric body. The dielectric and metal rectangular waveguides are connected by smooth waveguide transition (or junction).
The advantage of this device is the ability of impedance measurements of superconductors in the millimeter and sub-millimeter wave ranges.
A major shortcoming of the prototype, as well as other above-mentioned measuring resonators with whispering gallery modes, is a technical solution to the coupling units, which degrades performance of the measuring resonator as a whole. Electromagnetic coupling between an open dielectric waveguide and measuring open dielectric resonator is due to the presence of area in space, where the fields of a resonator and a waveguide overlap. In the case of open systems (due to the availability of coupling with space), this leads to two effects: 1) the field of the resonator is scattered by the waveguide, thereby reducing the eigen Q-factor of the resonator, 2) a traveling-wave field of dielectric waveguide is scattered by the resonator into space, thus reducing the efficiency of energy transfer into the resonator. These two effects appear the stronger, the closer are placed together the waveguide and the resonator. Consequence of the abovementioned effects is the deterioration of the signal-to-noise ratio of measurement system and the existence of systematic (methodical) measurement errors. The signal-to-noise ratio decreases due to the fact that the dielectric waveguide and the measuring resonator are moved away to a distance where the scattering of the resonator field can be neglected. In this case, the signal level is low due to the decrease of the coupling coefficient. In addition, the signal-to-noise ratio decreases due to the fact that the non-resonant radiation signal from the input waveguide and the useful signal from the resonator, which are comparable in the amplitude, interfere in the output waveguide.
At large distances the effect of resonant scattering can be neglected, but the coupling coefficient between the waveguide and measuring resonator thus becomes small, which in turn leads to a decrease in signal-to-noise ratio of the measurement system as a whole. Effect of non-resonant radiation from dielectric waveguide leads to electrodynamic connection between the input and output waveguides, which causes interference effects in the output waveguide between signal of non-resonant radiation and useful signal, which complicates the procedure of tuning the measuring resonator, and also reduces the signal-to-noise ratio. Effect of non-resonant radiation at large distances between the waveguide and the resonator can not be neglected, because power non-resonant radiation can be comparable to the power of the signal.